1. Field of the Invention
The present invention relates to a composition and a process for preparing a transparent conducting film on substrates of glass, ceramics, etc.
2. Brief Description of the Prior Art
The transparent conducting film is widely utilized as electrodes for liquid crystal display devices, electrochromic devices, EL, photosemiconductors, etc., and further as non-fogging exothermic resistors for automobiles, airplanes, etc.
It is known to prepare the transparent conducting film by coating a substrate with a solution containing an indium compound and a tin compound by dipping, spinner coating, brush coating, printing, etc., and heat-treating the coated substrate, thereby thermally decomposing the organic components and forming an oxide film of indium and tin.
The well known solution containing an indium compound and a tin compound includes (1) a coating solution containing a fluorine-containing indium compound such as basic indium trifluoroacetate, and stannic chloride, (2) a coating solution containing indium chloride and stannic chloride, (3) a coating solution containing an alkoxy indium or indium carboxylate, and an alkoxy stannous compound or stannous carboxylate, (4) a coating solution containing indium naphthenate and tin octylate, and (5) a coating solution containing indium nitrate dissolved in .beta.-diketone or a mixed solution of .beta.-diketone with other solvent and tin halide (stannous chloride, stannic chloride, stannous bromide, stannic bromide, stannous iodide, or stannic iodide) or tin nitrate or tin acetate.
However, as a result of investigations of characteristics of films obtained by applying these coating solutions onto soda glass substrates each having a silicon oxide film (thickness: about 2,000 A) on the surfaces and firing the substrates at 500.degree. C., substantial upper limit temperature giving no deformation to the soda glass, for one hour, it was found that these coating solutions had the following problems. The film obtained from the solution (1) had a high resistance, the one obtained from the solution (2) had a white turbid state and also had a high resistance, the ones obtained from the solutions (3) and (4) had a little lower but not satisfactory resistance then those of the films obtained from the solutions (1) and (2), and the resistance was liable to fluctuate greatly with a slight change in film-forming conditions (for example, a slight change in temperature and humidity at the site of coating, drying and calcining conditions for the coated film, etc.). That is, it was difficult to obtain a film of low resistance with stability. Furthermore, the film suffered from such disadvantages as low mechanical strength and high chances to damages.
The film obtained from the solution (5) had relatively better characteristics, but had still higher resistance. The resistance was liable to fluctuate greatly with a slight change in the film-forming conditions. The stability of the solution with time was poor, and the resistance of the resulting film was increased within 10 to 20 days after application even by using the coating solution at room temperature (23.degree. to 25.degree. C.).
It is obvious from the foregoing that the coating solutions so far known cannot produce a transparent conducting film having a good light transmission, a low resistance and good mechanical strength without any white turbid state with stability.